1. Field of the Invention
The present invention relates to a method and system for controlling a plurality of transmitters and, particularly, to a method and system for controlling a plurality of transmitters in a cellular mobile communication network.
2. Description of the Related Art
In typical cellular mobile communication network, a mobile station sends uplink packets, such as an uplink data packet, to a base station over the air and the base station forwards the received packets toward an appropriate upper layer, e.g. the Internet. To support efficient uplink packet transmission from multiple mobile stations in the cellular mobile communication network, the base station schedules the multiple mobile stations in order to maximize the total uplink data throughput in the cell while meeting a service requirement of individual mobile station.
As an example uplink packet scheduling system, FIG. 1 illustrates the Enhanced Uplink Dedicated Channel (EUDCH) system. In EUDCH, the mobile stations 101-1 and 101-2 are exchanging control messages with the base station 102 such as scheduling request message in uplink and scheduling grant message in downlink. A base station scheduler (i.e., a scheduler in the base station 102) assigns radio resource by transmitting scheduling grant messages while the mobile station can request the resource by transmitting scheduling request message (see 3GPP TS 25.309 V6.3.0 (2005 Jun. 28) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; FDD Enhanced Uplink; Overall description; Stage 2 (Release 6) and 3GPP TR 25.808 V6.0.0 (2005 Mar. 29) Technical Report 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; FDD Enhanced Uplink; Physical Layer Aspects (Release 6)).
One of problems of conventional technology of the Interest, such as EUDCH technology, is that a collision of packet transmission can occur when a mobile station fails to receive a scheduling grant message sent by the base station scheduler. An illustration of aforementioned problem is shown in FIG. 2. In this example, the base station 102 is scheduling two mobile stations. Initially, the mobile station 101-1 is transmitting its packets, and then the mobile station 101-2 requests for transmission. The base station scheduler decides to halt the packet transmission from the mobile station 101-1 in order to start packet transmission from the mobile station 101-2. However, a deactivation scheduling grant message for the halt sent to the mobile station 101-1 is failed to be received by the mobile station 101-1, while an activation scheduling grant message sent to the mobile station 101-2 is successfully received by the mobile station 101-2. Then, the data transmissions of the two mobile stations 101-1 and 101-2 collide each other as soon as mobile station 101-2 starts its transmission.
In the example illustrated above, the mobile station 101-1 fails to receive the deactivation scheduling grant message due to hazardous downlink wireless channel condition or the like. In typical scenario of a large number of scheduled mobile stations and the unpredictable wireless channel condition of each mobile station, a perfect reception of scheduling grant message is non trivial task. Then, as a result of miss reception of the deactivation scheduling grant message, the data transmission of respective mobile stations collide each other which then results in the loss of transmitted data.
In the example illustrated above, the base station 102 could send the activation scheduling grant message to the mobile station 101-2 after detecting that the mobile station 101-1 has stopped its transmission. In this way, it could be possible to avoid the packet collision between the two mobile stations. However, the base station has to detect the mobile station 101-1 stopped its transmission before it sends activation scheduling grant message to mobile station 101-2. In order to allow the detection, there should be a time period when none of two mobile stations are transmitting data packets. And hence this would create another problem of resource under utilization during the transition period.